Updated: March 31, 2016
Me haz noticed that, sometimes, quite often, the initial Windows Update discovery process takes a long time to complete. It depends on how many updates are available, how long it's been since the last full patch cycle, and what kind of hardware you have. But often as not, it takes a good 60-90 minutes to give you the list of updates you can install. During that entire time, svchost.exe sub-process of the WU executable eats a full CPU core worth of processing power. After that, the actual install is usually faster, if not really fast.
This rather inconsistent and somewhat annoying behavior of the WU discovery prompted me to try to find a solution to this issue. Searches, queries and forum questions led me to two rather long and exhaustive threads on Wilders and DSLReports, offering a wealth of suggestions and options. With a spare box awaiting testing, I set about exploring.
In more detail, whether you've just rebooted, manually initiated a check for updates, or your system runs its periodic refresh, WU takes a very long time to retrieve and present the results. In some cases, it can be many hours. The CPU is quite busy at that time. For example, one of my desktop would take 25% CPU for about 80 minutes before giving back a list of some 50 odd updates. I've read of situations where it takes days to finish the sequence.
The actual installation is somewhat faster and more consistent, even though you often won't see any percentage change in the progress bar. Then, the installation will be as good as your CPU and disk can process the patches. After a reboot, the subsequent checks will be significantly faster, but never immediate. Taking the previous example, once the 50+ updates were processed and installed, the next check only lasted about four minutes. However, while the number of updates definitely seems to impact the times, it is not the only factor. And that makes the whole thing rather unpredictable.
Well, there are many reasons and workarounds available. The first and most immediate one is to download the latest version of the WU agent, which could potentially speed things up a little. This is probably the first check you could try.
There is a long list of patches that have been released to help tackle the issue of update searches and installations being slow, accompanied by high CPU usage. Specifically, one of these is KB3102810. However, there are several other patches of the similar nature, released both before and after this one. For instance, KB3112343, KB3138612.
To complicate things a little further, these patches also seem to bundle some Windows 10 upgrade goodies, which makes them potentially unwanted, as well as complicates things when it comes to solving slow updates. Plus, there's the question, which one should you try? The latest? The oldest? Well, mathematically, you should start with the oldest, because you can always upgrade, or revert, if you encounter a regression.
I tested the three KB listed above, with and without reboots in between, a total of six permutations. I must say that I have not been able to discern any repeatable pattern of improvement or speed. On this particular test box, the CPU still remained busy at 100% most of the time, and the searches took a very long time. Then, I expanded the testing to another system, with somewhat better results, but there was also quite a bit of variation in between different patches, too. All I can say, whatever Microsoft had in mind probably only works under very specific conditions. Perhaps even they aren't really sure what might be wrong or how. Anyhow, the KB might work. Or they might not. 'Tis a happy gamble. Another patch that might work is KB3139852, and it might actually do the necessary magic, but we will only be able to ascertain that in the coming months.
This is a short article, with an unsatisfying ending. There are several official patches that claim to fix the slow update and search and high CPU usage. But whether they work in any smart or consistent manner is beyond me. I was able to achieve only partial results with this approach. I still don't have a formula that tells me exactly under which conditions the WU plays well and under which is crawls. The last update may promise Nirvana, we shall see yet.
You might try these patches, you have nothing to lose. However, start with the oldest release and then move forward in time. Make sure you only perform one change at a time so you have a comparison baseline. Then, imaging and backups are always good. Last but not the least, in the coming weeks and months, we will test some third-party tools that promise speedy and fun results with Windows updates. But that's a secret for now. We shall reconvene, stay tuned for updates.